Pulping system with impregnation digester having liquor recirculation and cleaning loop

ABSTRACT

A PULPING SYSTEM AND PROCESS FR SUCCESSIVE IMPREGNATION, COOKING AND DEFIBERIZING OF RAW MATERIAL IN A CONTINUING FLOW THEREOF UNDER CONDITIONS OF TEMPERATURE AND PRESSURE WHICH DICATE A GENTLE AND EFFECTIVEE SEPARATION. THE SYSTEM FEATURES, IN CINNECTION WITH THE IMPREGNATION VESSEL EMPLOYED, A RECIRCULATION SYSTEM FOR MAINTAINING IN THE IMPREGNATOR A BATH OF CONDITIONING LIQUID OF FUNIFORM TEMPERATURE AND CONCENTRATION. THE RECIRCULATION SYSTEM INCLUDES A DISCHARGE CONDUIT WHICH CONNECTS TO THE IMPREGNATION DIGESTER BELOW THE LEVEL OF BATH THEREIN AND CARRIES THE LIQUID TO BOTTOM PORTION OF A FLASH TANK. A DELIVERY CONDUIT CONNECTED TO THE BOTTOM OF THE FLASH TANK CARRIES THE LIQUOR THROUGH A PUMP AND HEAT EXCHANGER BACK TO THE IMPREGNATION DIGESTER BELOW THE BATH LEVEL. WHITE LIQUOR CAN BE ADDED TO THE LIQUID IN THE DELIVERY CONDUIT BETWEEN THE FLASH TANK AND THE PUMP. A SECONDARY RECIRCULATION OF THE LIQUOR IS ACCOMPLISHED BY USE OF A CONDUIT CONNECTED TO THE DELIVERY CONDUIT DOWNSTREAM OF THE PUMP WHICH CARRIES SOME OF THE LIQUOR THROUGH ANOTHER HEAT EXCHANGER AND INTO THE TOP PORTION OF THE FLASH TANK. CONTAMINATED STEAM IS REMOVED FROM THE TOP OF THEE FLASH TANK.

June .1974 J. R. STARRETT 3.314.662

PULPING sY-sTEu WITH IMPREGNATION DIGESTERHAVING- LIQUOR REOIRCULATION AND CLEANING LOOP Original Filed Sept. 8, 1969 RPENTINE CONDENSER TURP NTIN STORAGE 20 KNOTT R TO WEAK 60 B-LKLIQ/ TANK so PSIG LOW PRESSURE STEAM TO DOWNSTREA PROCESS -M/VE/VTUI? JAMES R.v STARRETT By 1 $1W I75 PSIG STEAM ATTORNEY LIQ United States Patent US. Cl. 162-19 Claims ABSTRACT OF THE DISCLOSURE A pulping system and process for successive impregnation, cooking and defiberizing of raw material in a continuing flow thereof under conditions of temperature and pressure which dictate a gentle and effective separation. The system features, in connection with the impregnation vessel employed, a recirculation system for maintaining in the impregnator a bath of conditioning liquid of uniform temperature and concentration. The recirculation system includes a discharge conduit which connects to the im pregnation digester below the level of bath therein and carries the liquid to bottom portion of a flash tank. A delivery conduit connected to the bottom of the flash tank carries the liquor through a pump and heat exchanger back to the impregnation digester below the bath level. White liquor can be added to the liquid in the delivery conduit between the flash tank and the pump. A secondary recirculation of the liquor is accomplished by use of a conduit connected to the delivery conduit downstream of the pump which carries some of the liquor through another heat exchanger and into the top portion of the flash tank. Contaminated steam is removed from the top of the flash tank.

This is a continuation of applicants co-pending application for United States Letters Patent Ser. No. 856,039, filed Sept. -8, 1969, now abandoned.

This invention relates to a new and improved chemical pulping process. It also provides a unique pulping system by means of which the invention process may be optimally practiced. The invention is particularly advantageous for use in achieving high quality Kraft pulp from raw wood chips and will be illustratively described with this in mind. However, it should be obvious that neither the use nor the application of the invention is so limited.

There have been many methods and systems directed to Kraft pulping. However, basic problems continue to perplex the pulping industry in respect to their eflorts to minimize the usage of chemicals and the requirements for a heavy supply of fresh water and steam. By minimizing the needs in the areas noted, one achieves substantial reduction in the space required for a pulping installation as well as a reduction in the capital cost of the necessary equipment. This is apart from the obvious savings in these materials and the handling thereof. Moreover, by providing a system and process wherein there is more ready and more efiicient control of liquors, their temperatures, their use and re-use thereof, the exposure of the raw material which is pulped generates less possibility of degradation, and movement thereof can be achieved at higher consistencies. The net result which is achieved by the present invention as distinguished from the systems and Patented June 4, 1974 methods of the prior art is a cleaner, better quality and more economical production of pulp. The noted advances and other improvements inherently afforded by the invention will become more obvious from the following description.

It will be recognized that the invention has many aspects and one particular aspect thereof is a distinct improvement over the prior art as exemplified by the US. Patents, Tomlinson 3,258,390 and Bryce 3,448,002. These patents teach, in connection with an impregnator, means for a direct line cycling of spent liquor from and to the impregnator by way of a heat exchanger. Note, however, that what is delivered to the impregnator is actually a mixture of spent liquor and fresh liquor. By reason of the single pass direct flow of the liquor the system inherently lacks the ability to exercise full control of the quality or chemical concentration of the bath of liquor required in the impregnator. This has been determined in eliorts to commercially employ this prior art practice, the results of which are in distinct contrast to the potential for full control of the level, temperature and concentration of conditioning liquid as enabled by the present invention.

It is therefore a primary object of .the invention to provide an improved and unique pulping process and a system for achieving the same which is economical to fabricate, more efiicient and satisfactory in use, adaptable to a wide variety of applications and unlikely to malfunction.

Another object of the invention is to provide an improved process particularly advantageous for Kraft pulping and characterized by maximal effective usage of minimal amounts of white liquor.

A further object of the invention is to provide an improved pulping process characterized by a maximal usage of waste steam.

An additional object of the invention is to provide an improved pulping process characterized by a gentle, presteaming, followed by impregnating in a liquor bath wherein liquor is circulated and recirculated at a controlled reaction temperature whereby to effect an optimal active chemical absorption in the impregnating process.

Another object of the invention is to provide an improved pulping process and system for achieving the same characterized by means for impregnating raw wood chips in a bath from which weakened liquor is strained, concentrated, reinforced and returned in a manner that on reentry to the bath it has the required reaction temperature of the bath for optimal impregnation.

A further object of the invention is to provide an improved pulping process and system for achieving the same characterized by means for evaporating excess water from portions of the system without adverse effect in the operation thereof on raw wood chips.

Another object of the invention is to provide an improved pulping system and process characterized by independent separately, controllable circulation loops enabling precise temperature control of the operating chemicals.

An additional object of the invention is to provide an improved pulping process characterized by pre-steaming or pre-conditioning, impregnating and cooking raw wood chips in the pulping process so as to positively treat each particle in the same gentle manner and expose it to the same reaction media in the same time period.

A further object of the invention is to provide an improved pulping system and process characterized by means enabling an effective high consistency movement of the pulp material.

An additional object of the invention is to provide an improved pulping process and system possessing the advantageous structural features, the inherent meritorious characteristics and the means and mode of operation herein described.

With the above and other incidental objects in view as will more fully appear in the specification, the invention intended to be protected by Letters Patent consists of the features of construction, the parts and combinations thereof, and the mode of operation as hereinafter described or illustrated in the accompanying drawings, or their equivalents.

Referring to the drawings, wherein is shown schematically one but not necessarily the only means and manner of achieving the present invention,

FIG. 1 is a generally schematic diagram illustrating the invention system and process for pulping raw wood chips.

The invention system here illustrated includes, in series related sequence, a vertically oriented pre-steaming tower 10, feeder 11, rotary valve 12, an impregnating digester 13, a cooking digester 14, surge tube 15, blow tank 16, pump 17 and knotter 18.

As schematically shown, raw wood chips are delivered to the open top of the tower by way of a hopper 19. This establishes and maintains a vertical column of chip material which is first conditioned by pre-steaming in the course of its movement through and from the tower. The nature of the column is such to effectively seal the top of the tower and enable the establishment therebelow of an above atmospheric pressure, as will be described.

Opening to and projected laterally of the base of the tower 10 is a housing 20. The discharge from the housing 20 is bridged by the rotary valve 12. The latter is a rotary pocket-type valve of a conventional nature Which is well known to provide a vapor lock. Thus, with the chips forming a seal at the top of the tower and there being a vapor lock at the discharge from the housing 20, the housing 20 is pressurized. The housing has for this purpose a steam dome 21 connected by a conduit 22 to receive waste steam generated at the blow tank 16. Also included in the housing 20 is the screw feeder 11, the entrance end of which projects interiorly of the tower to lie immediately below the incoming chip column.

The waste steam directed inwardly of the housing 20 by way of the steam dome 21 is circulated in the housing and along the feeder 11 into the base of the tower. From here the steam continuously moves upwardly of the tower to fully disseminate through the column of chips. The invention provides that the pressure produced by the steam at the base of the tower is approximately .75 to 1 p.s.i.g. and the steam temperature between 215 and 220 F. Thus, the steam applied at the base of the tower and through the housing 20 is of a nature that as it moves upwardly it will heat the moisture in the chips and the chips per se to a boiling point just sufiicient to drive out contained air and initiate separation of cellulose from moisture, contaminants, and acids. Screens are provided at the base of the tower through which free fluids are removed as the pre-steamed chips flow down to the screw feeder 11 and moved in continuing flow through the pressurized steam environment of the housing 20.

The time of this described pre-steaming process is carefully controlled to take from about three to eight minutes, depending on the chip materials. A five minute process as described has generally been found to be optimal.

Particular note should be taken of the fact that there is no pressure on the top of the chip column. Therefore there is no means to prevent the free driving from the chips of the contained air and some non-condensables such as turpentine, Which is a free and natural consequence of the pre-steaming system and process.

As the so-conditioned chips are metered from the tower 10 by screw 11 through the housing 20, they are gently transferred by way of the rotary valve 12 to the higher vapor pressure of the impregnating digester 13.

At this point it must be noted that the pre-steaming or or pre-conditioning that has been effected provides for a gentle, gradual and extended uniform heating of the raw wood chips, gently freeing them of air Without any significant applied pressure and softening the physical structure of the particles to facilitate the presentation of undamaged, preconditioned, raw wood chips freed of undesirable elements and in an optimal condition to absorb the cooking liquor in the impregnator.

The impregnating digester is an M & D type impregnator and shown to be a vertically inclined tube divided longitudinally on its axis by a partition 24. The latter terminates short of the ends of the tube to provide thereabout a continuous loop passage including an upper segment 25 and a lower segment 26. The partition, as may be seen, also serves as a base for an endless conveyor device 27.

Referring to the drawing, the impregnator tube 13 is substantially filled with a suitable cooking liquor (pulping chemicals) to a level which is immediately below the connection into the segment 25 of the delivery conduit 28 leading from the rotary valve 12. Thus, the conveyor 27 will pick up the delivered chips immediately on entrance to the impregnator, just above the liquor level, and immediately drag them into and through the conditioning bath of liquor there provided. The conveyor moves the chips around the bottom of the partition and up the lower segment of the passage 26 to the point Where the chips are lifted from and clear of the liquor. As this occurs, the chips will drain of free spent liquor and then be dropped through the impregnator discharge opening for delivery to the inlet of the cooking digester 14 by way of the conduit 30. It will be noted that the spent drained liquor will fall to the adjacent top level of the liquor bath in the lower segment 26.

Referring to the impregnating digester 13, the upper or dome portion thereof above the liquor level is maintained at a pressure of p.s.i.g. and temperature of 367 F. Attention is directed to the fact that a source of fresh steam at p.s.i.g. is available for delivery by way of a line 32 having a valve controlled communication with chip delivery conduit 28. Suitable sensing means in the top of the tube 13 are arranged to provide for the control of a supplemental inflow of steam from line 32 by way of conduit 28 to maintain the prescribed temperature and pressure in the top of tube 13. Since the means for so doing is within the skill of a mechanic in the art, the details thereof are not described. In contrast, while 150 p.s.i.g. and a temperature of 367 F. is maintained on the level of the cooking liquor, the liquor bath is maintained at a temperature of about 300 F.

Attention is directed to the fact that, to maintain the conditions of the bath, cooking liquor is inroduced by way of a fluid injector, nested in partition 24, and communicating with segment 25 in a position spaced immediately below the required level of the bath, at a point designated as 33. Similarly at the opposite point, to the underside of the partition, cooking liquor of low active chemical concentration is withdrawn from the impregnating digester through a strainer covered passage nested in partition 24, and communicating with segment 26, shown to open at 34 from the partition.

In the practice of the invention an active concentrated liquor supply is directed into the bath at 33 immediately below the established level and immediately following the entrance point of the preconditioned steamed chips. Here it has its maximum active concentration and is at the temperature to react most effectively with the chips at their temperature.

Of importance is the system for providing and extract ing liquor in respect to the impregnating tube 13'. Leading from the digester tube 13 at the strainer 34 is a discharge line 35 under the influence of a pump 36. On the discharge side of the pump 36 the line 35 includes a check valve 37. However, prior thereto, between the valve 37 and the discharge side of the pump 36, the line 35 communicates with a branch line or conduit 38 leading to a flash tank 39. Extending from the bottom of the tank 39 is a conduit or delivery line 40 in which there is first a check valve 41 and immediately following an interconnection therewith of a conduit 42. The latter incorporates a pump 43 for delivering to the system a supply of fresh white liquor (concentrated pulping chemicals) from a suitable source, at a controlled rate governed by a common flow control loop. Immediately beyond its interconnection and communication with the conduit 42 the line 40 includes a pump 44, after which it is then branched into two interconnected conduits 45 and 46. The branch 45 is looped to connect back into the top of the flash tank 39, passing in the process through a heat exchanger 47. The exchanger 47 is fed with steam by way of a branch from line 32 to elevate the temperature of the fluid from tank 39 and any accompanying fresh liquor above the corresponding flash point of fluid at the vapor pressure in tank 39. Included in the branch from the steam line 32 is a control valve which is governed by suitable temperature sensing means provided in line 45 following the heat exchanger to insure that the steam here provided is adequate to insure that any fresh liquor or impregnation fluid that is fed through the branch 45 back to the flash tank 39 has the appropriate required super heated temperature.

The branch 46 connects into line 35 beyond the check valve 37, from which point the line 35 continues to complete a loop back to the impregnating digester 13 where it opens at 33 to the upper segment 26 of the impregnator tube passage. In the process, the line 35 passes through a second heat exchanger 48 prior to reaching the impregnating digester. The heat exchanger 48 is optionally furnished steam, if required, from line 32 or coolant water from a suitable source. As will be further described, the device 48 will function primarily for cooling purposes.

Attention is directed to the fact that the flash tank 39 has at its top a discharge line 49 for removal of low pressure contaminated steam.

The primary objectives and achievements of the cooking liquor circulation and supply system for the impregnating digester 13 are to insure the maintenance of the required reaction temperature specified for the impregnating procedure and to provide that the concentration of the cooking liquor when first contacted by the chips is optimal.

Accordingly, in operation of the circulation and supply system, as chips are moved through and impregnated in the bath of cooking liquor and moved clear to discharge from the impregnating digester 13, the relatively weakened and somewhat contaminated liquor at the discharge end of the bath is pulled out of the tube 13 at 34 under the influence of dome vapor pressure and of the pump 36. Excess weakened liquor and water (fluid) is thus bled from the impregnating vessel 13 via line 35 and line 38 which contains a flow control valve that is actuated by a liquid level detector and control device and discharged into the bottom part of flash tank 39. It will be recognized, of course, that the demand for fresh liquor in the system is governed by the amount of chips delivered to the tower and their rate of movement. In correspondence therewith, the pump 43 will be energized to deliver a controlled amount of fresh white liquor to the tank delivery line 40. This will come under the influence of the pump 44 which during normal operation will draw from the flash tank 39 the weak liquor delivered from the impregnating digester 13. In the process the pump 44 will mix the weak liquor with a limited amount of fresh liquor and drive it back to the tank 39 by way of line 45, to pass through a heat exchanger 47 in this process. At the heat exchanger suflicient heat is added to provide the desired temperature. The flash tank pressure is maintained in the tank 39 at about 60 p.s.i.g., a pressure only slightly above that to produce the temperature of the cooking liquor as required in the impregnating digester. The mixture of weak liquor and fresh liquor moving through the loop to the flash tank 39 is heated at the exchanger 47 to a level just above its flash point corresponding to the pressure at which the tank 39 is maintained. Thus, as the heated circulated mixture enters the tank 39 flashing occurs, in the process of which excess moisture and contaminants from the weak liquor and excess moisture in the fresh liquor is driven off as a low pressure steam to leave the combination of the drained liquor and fresh liquor in an optimal reaction condition. Moreover, the essentially fresh chemical so provided will normally be at a temperature slightly higher than that required in the impregnator tube 13. The flash tank fluid thus flashed and concentrated is then drawn off under the influence of the pump 44 to be pressure flowed through the branch 46, to the extent permitted by the flow control in that branch. This chemical will then be delivered to the main return line 35 to flow therethrough and through the heat exchanger 48 to the tube 13. Of course, the amount of flow through the branch 46 is under suitable control dictated by the fresh white liquor demand in accordance with wood flow. It the temperature of the concentrated supply has to be lowered at the heat exchanger, coolant water is passed therethrough rather than steam and vice versa. The adjustment at this point is relatively minor. In any event, the fresh supply is made up to the required temperature and at an optimal concentration, including a substantial amount of recaptured liquor from which excess moisture and contaminants have been removed at the tank 39. It is noted that the efliciency and the nature of the procedure is such that a very nominal amount of fresh liquor need be or is added during an extended operating period of the impregnating digester 13.

It should be here noted that in this system the flash tank 39, being completely independent of the impregnating vessel 13, will permit flash tank operating pressures and temperatures in any range either above or below the impregnator bath temperature and its corresponding pressure; or the impregnator dome pressure and its temperature. It should be recognized that lower flash tank pressures require that more heat be added by exchanger 48 while higher flash tank pressures require more heat extraction by exchanger 48. Also flash tank pressures above impregnator dome pressure will require higher discharge head (pressure) pumps 36, 43 and 44. By use of the independent flash tank, it is feasible to operate the flash tank above the impregnator dome pressure thus permitting the flash steam to be used to maintain dome pressure.

An incident of the operation of pump 36 is that by appropriate control thereof this circulating pump can be caused to produce back pressures to backwash the fluid withdrawal strainer at 34. This insures the continuing recirculating pattern of the chemical for the impregnating digester.

Thus, to this point the chips have first been subjected to pre-steaming at a temperature which is low to avoid burning the chips or damaging the fibers and to prevent pre hydrolysis, that is to prevent premature loss of chip moisture and wood acids which, if lost, under such conditions would break down the cellulose. Also, in the impregnating digester 13, these gently pre-steamed raw wood chips which have been placed in a most receptive condition and in a manner to avoid fiber damage and degradation are here equally subjected, substantially immediately on entry, to a substantially constant temperature liquor. The liquor is both at a temperature and concentration at the beginning of the impregnating cycle to produce optimal and most complete impregnation. The chips remain in a high consistency form as they move through the bath and are then drained of spent and excess liquor as they are discharged from the impregnator to the top of the cooking digester 14 in an environment of 150 p.s.i.g. and 367 F. The discharge from the tube 13 is by way of gravity and through the open conduit 30. Accordingly, the cooking area of the tube 14 is obviously under the same conditions of temperature and pressure, that is 150 p.s.i.g. and 367 F.

It must be recognized that it is a factor in effecting the optimal form of the chips leaving the impregnating di gester that the temperature of the bath in which the chemical impregnation takes place dictates a pressure effect of 50 p.s.i.g. This is in contrast to the 150 p.s.i.g. pressure which exists on the level of the liquor bath. This is significant in avoiding overheating and degradation of the fibers while providing for an adequate and most effective accommodation of the chemical by the fibers making up the chips.

Referring now more particularly to the cooking digester 14, this is a hollow tube including a mid-feather in the form of a partition 51 similar to the partition in the impregnating digester 13. The partition forms thereabout a continuous cooking passage, as in the tube 13, including an upper longitudinal segment 52 and a lower longitudinal segment 53. At the upper end of the segment 53 there is a discharge opening through which cooked chips are directed from the tube 14 to the surge tube by way of a short open conduit 54. Mounted for continuous movement about the partition 51 is an endless conveyor 55 which at the top of the segment 52 picks up the incoming impregnated chips as they are delivered from the tube 13. The conveyor carries the impregnated chips down the one side of the partition 51 and up the other.

All the while, as they are moved through the pressurized environment of tube 14 the chips are cooking. A portion of the fluid (cooking liquor) which enters the tube 14 with the chips is inherently extracted, under the conditions of temperature and pressure described, and flows to form a shallow pool at the lowermost end of the cooking digester, through which pool the chips must pass in their movement to discharge. In this case the partition 51 has in its lower end an opening 56 to a drain passage through which the extracted excess in the form of a black liquor is directed to a receiver 57. A line 58 connects the receiver 57 to the top of a flash tank 59. A strainer is also provided for a discharge to the receiver 57 from the bottom of the cooking tube 14.

In the flash tank 59 the high temperature black liquor from the receiver 57 is flashed to reduce the fluid temperature and increase the fluids dissolved solids consistency and in flashing steam resulting is directed to the blow tank 16 which is maintained under 2% p.s.i.g. pressure and becomes useful as a steam source for the presteamer. The high solids content black liquor created in tank 59 is moved by a pump 60 to a weak black liquor storage tank for chemical recovery. The cooked impregnated wood chips which are free of excess liquor pass from tube 14 to move through the surge tube 15. The latter is a small pressure vessel, the pressurized condition of the described system being maintained therein and thereby. As the optimally cooked and conditioned chips enter they are naturally at a maximum consistency, having been drained and stripped of excess moisture in the tube 14.

Nozzles are incorporated in the surge tank 16 to apply a cooling jet inflow of fluid to the moving cooked chips. In passing the chips are thereby reduced as to their temperature and their consistency is reduced also. In the lower portion of the surge tube there are suitable conventional agitating means. Due to the high consistency and completely cooked condition of the chips the agitating means easily reduces them to individual fibers in the course of their passing from the surge tank. The form of the agitator is not itself detailed since the details themselves play no part in the invention. The important fact is that it only requires a turbulence inducing medium of any conventional character to defiberize the chips at this point. This is due to the chip condition achieved by the invention system and process.

Suitable flow control valves are provided at the discharge of the surge tube 15 which are of a character to maintain the pressurized condition of the preceding portions of the system while discharging to the blow tank 16 the fiberized pulp deriving from the wood chips. The blow tank, of course, is at a pressure substantially lower than p.s.i.g., usually 2 /2 p.s.i.g.

Blow tank 16 also is conventionally fitted with nozzles to supply therein fluid for diluting and cooling the pulp fibers and an agitator for mixing in the fibers this dilution fluid. As the hot fiberized chips enter the blow tank 16 they are flashed to remove moisture. One thereby provides the steam for the pre-steamer, the excess being used in various manner for heat recovery.

The fiber stock is diluted and cooled in the blow tank 16, is drawn therefrom by the pump 17 and is delivered thereby to a knotter 18. The latter is of the vibrating screen type. Here any uncooked or unfiberized chip particles which might remain are picked up by the screen and returned by way of a line 62 to a screw press 63. At the press these rejects are both dewatered and reduced prior to their re-entry to and re-passage through the conditioning and cooking system as described. The rejects are re-introduced to the pre-steamer tower 10.

In the main, however, the system produces essentially long, bright, clean fibers to the extent within the capacity and character of the applied wood chips. These fibers pass the knotter for washing and storage or immediate delivery for end product use. The fiber production is essentially the maximum possible from the particular raw material which is first introduced.

As may be seen from the drawings, the system provides that there be drawn from the high temperature tubes noncondensibles such as turpentine. Such is moved to a condenser 65 for recovery of the by-products, the latter being moved from the condenser to storage.

The invention system described is exceedingly simple to install and its use quite economical, particularly considering the character and quality of the end products produced thereby. As seen, it contemplates a low fresh water demand since there is a very low direct stream consumption. The major portion of the heating load requires only the use of flashed steam, deriving to a large extent from moisture entering the system in the chips or liquor. Moreover, much of the heating is indirectly achieved.

Further, in the practice of the invention, water is evaporated from the cooking liquor in what might be considered a relatively isolated or independent evaporation sub-system, such as the one involving flash tank 39. Within this system, depending on demand and valve control V in line 46 following the loop including items 44, 45, 47, 39 and 40, contaminated liquor and fresh liquor can be cycled and recycled to achieve desired concentration and temperature. The arrangement is such to increase the effective concentration of original and drained cooking agents to a degree and quality the chemicals can be used and re-used, being returned each time at the proper reaction temperature and active chemical concentration. The net result is a low chemical consumption. It is noted that the sub-system for handling and circulating the cooking and conditioning liquor may be operated at any required temperature depending on the materials and application without any effect on the operation of the impregnating digester per se. Thus, the reconstitution of the cooking liquor can be effected and it can be mixed with measured quantities of fresh liquor externally of the impregnating digester and water extracted from the fresh liquor with no adverse elfect on the impregnating pressure, temperature, chemical activity or impregnating time.

The independence of the sub-system above referred to can be best seen with reference to the following table which reveals two extreme sets of operating conditions for the invention system as illustrated.

Operating conditions Conditions of described Cond. Cond. Item embodiment A" B" Presteamer (20) pressure, p.s.i.g. 0.75 0.75 0.75 Impregnator and cooker pressure, p.s.i.g 150 150 150 Impregnator bath temperature,

F 300 300 300 Cooker (l4) temperature, F-.- 367 367 367 Blow tank (16) pressure, p.s.i.g 2.5 2.5 2. 5 Steam supply pressure (32),

p.s.i.g 175 175 250 Flash tank (39) pressure, p.s.i.g 60 3 175 Flash tank (59) pressure, p.s.i.g. 2.5 2.5 2. 5 Pump (43) discharge pressure,

p.s.i.g 75 18 190 Pump (44) discharge pressure,

p.s.i.g 200 200 200 Pump (36) discharge pressure,

p.s.i.g 190 190 190 As seen from the respective sets of operating conditions, subject only to providing the adequate level of steam pressure in line 32, one may, for example, vary the operating pressure established in the flash tank 39 from 3 p.s.i.g. to 175 p.s.i.g. to provide therein a desired effect on the weak liquor extracted from the impregnator and the mixed fresh liquor delivered thereto by pumps 43 and 44. When operating the flash tank 39 at the higher level, one will of course require the pump 43 to operate at the higher discharge head indicated. Thus, by simply selecting a desired flash tank operating pressure and temperature, one can independently control the liquor of the system to achieve the desired chemical reaction and condition thereof without degradation of the fibers for the purpose intended and utilizing thereby a minimal amount of liquor to maximum effect. An attendant benefit is the ability to provide in the process the fullest utilization of waste steam. It will be noted that where the condition B of the above table exists, the steam flashed at the tank 39 can be applied to even maintain the pressure and temperature in the impregnator and cooker, as well as to provide the same for use in operating the heat exchanger 48.

Accordingly, the significant advantages of the independently operating liquor recirculating loop of the system become self-evident.

Of course, the temperature and pressure in the flash tank 39 can be set at any desired level by anyone versed in the art, utilizing conventional pressure release or control devices. The contaminated fluid or liquor with attendant fresh liquor, if any, is cycled to the flash tank through heat exchanger 47 to produce a predetermined flash point of the fluid in correspondence with the pressure and temperature at which the flash tank operates. Thus, clean fluid relieved of excess moisture leaving the flash tank may be at any desired temperautre or pressure, uncontrolled by the conditions existing in the connected impregnator or any other connected vessel.

Another aspect of importance is that, as previously noted, the materials are maintained at a high consistency to the surge tank. This minimizes the requirement for diltion fluid prior to the stock being blown to the tank 16.

Also, the independent liquor circulation loop for the impregnating digester is such to enable precise temperature control without involved equipment or procedures, a very important factor in achieving uniformity of end product. Additionally, as provided throughout the system, each particle of raw material is at all times similarly treated with the same reaction media, in the same gentle manner and for the same time intervals.

In respect tot he cooker portion of the system, the flash tank and level control loop as here provided insures the continuing extraction from the bottom of the cooker 14 of dissolved wood solids and spent chemicals and Water.

7 is thus provided a device of the character described possessing the particular features of advantage before enumerated as desirable, but which obviously is susceptible of modification in its form, proportions, detail construction and arrangement of parts without departing from the principle involved or sacrificing any of its advantages.

While in order to comply with the statute the invention has been described in language more or less specific as to structural features, it is to be understood that the invention is not limited to the specific features shown, but that the means and construction herein disclosed comprise but one of several modes of putting the invention into effect.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Pulp processing apparatus including a pressurized vessel accommodating a bath of conditioning liquid for the impregnation of raw pulp materials, said vessel having an inlet thereto and an outlet therefrom and including means in connection therewith for introducing the raw materials into said bath by way of said inlet, and means for continuous movement of the raw materials through the bath and for removing the materials, impregnated thereby, from said vessel by way of said outlet, characterized by means defining a flow path exterior to said vessel one end of which is connected to said vessel to receive conditioning liquid which has been diluted and contaminated to achieve the form of a weak liquor and the other end of which connects into said vessel adjacent said inlet, means for extracting said liquor and inducing the movement thereof through said flow path, means incorporated in said flow path defining a recycling loop for said liquor, said flow path including, in connection with said recycling loop, means for separating contaminants and excess moisture from said liquor and returning the resultant liquid, which has been cleansed and/ or concentrated, to said flow path to move down stream of said recycling loop and back to said vessel for re-use, and first heat exchanger means incorporated in said flow path down stream of said recycling loop to raise or lower the temperature of the liquid in said flow path, as and when required, prior to its return to said vessel.

2. Apparatus as in claim 1 characterized by said recycling loop including means to elevate the temperature of said weak liquor prior to entering said separating means.

3. Apparatus as in claim 1 characterized by said other end of said flow path being arranged to open into said vessel in an area thereof which will lie immediately be neath the level of the contained bath whereby to insure that the incoming raw material will be met with conditioning liquid which is uniformly clean and concentrated.

4. Apparatus as in claim 3 characterized by means for pressurizing said vessel and subjecting the upper level of the bath in said vessel to a temperature environment equal to or significantly greater in degree than that of the body of said bath.

5. Apparatus as in claim 1 characterized by means for directing controlled amounts of fresh conditioning liquid into said flow path and into said recycling loop for mixture thereof with said recycled liquor and relief therefrom of excess water.

6. Apparatus as in claim 1 characterized by said means for separating contaminants being a flash tank and ineluding a second heat exchanger means and there being valve means to provide for a controlled movement of the weak liquor through said recycling loop to pass first through said second heat exchanger means and then to the upper part of said flash tank, the latter of which includes means wherein the excess moisture and contaminants are removed from said flash tank.

7. Apparatus as in claim 1 wherein said means for separating contaminants comprises a flash tank and said recyling loop includes a conduit for recycling liquor down stream of said flash tank to the upper part of said flash tank.

8. Apparatus as in claim 1 including means for introducing limited amounts of fresh conditioning liquid into said flow path and said recycling loop providing a closed circuit for relieving said fresh liquid of excess water prior to routing of said fresh liquid to said vessel by way of said flow path.

9. Apparatus as in claim 1 characterized by means for applying in said vessel, above said bath, a vapor of high pressure and relatively high temperature and another vessel for cooking impregnated raw pulp materials having an inlet receiving materials from the outlet of the first said pressurized vessel and said vessels being in free and open communication whereby said cooking vessel will have a vapor therein at a pressure and temperature corresponding to that of the vapor above the bath in said impregnation vessel.

10. Pulp processing apparatus as in claim 1 characterized by means in connection with said means for introducing the raw materials into said bath for pressurizing said materials while subjecting the same to a flow of steam at a temperature approximating the boiling point of their contained moisture whereby to gently condition the materials to achieve an optimal impregnating reaction on contact with the conditioning liquid of said bath.

11. Pulp processing apparatus as in claim 1 characterized by a cooking vessel having an inlet and an outlet, its inlet being in open communication with the outlet of said impregnating vessel and there being means for maintaining a vapor bath in the interior of said cooking vessel which corresponds in temperature and pressure with vapor maintained in the first said pressurized vessel above the bath of conditioning liquid therein.

12. Pulp processing apparatus as in claim 11 characterized by means in connection with said cooking vessel for drain therefrom of liquor extracted from the impregnated material in the course of movement therethrough and further means in connection with said cooking vessel to flash moisture from this drained liquor and to direct the same for further use in said system.

13. Pulp processing apparatus as in claim 11 including a blow tank in connection with said cooking vessel and interposed between said cooking vessel and said blow tank there being means to receive the high consistency relatively dry material which will issue from said cooking vesel to fiberize and fluidize the same so as to discharge it to said blow tank in a condition that steam will flash therefrom.

14. Pulp processing apparatus as in claim 1 applied to raw wood chips characterized by a cooking vessel in connection with said pressurized vessel outlet to receive impregnated material therefrom and cook the chips, a blow tank in following relation to said cooking vessel, means for directing the cooked chips from said cooking vessel to said blow tank and in the process thereof to defiberize said chips and means for directing steam evolving from operation of said apparatus to pre-condition the chips in the course of their delivery to the inlet of said pressurized impregnating vessel.

15. In a pulp processing system, separate vessels for the impregnation and cooking of a pulp material, said impregnation vessel having a pulp materials inlet and pulp materials outlet, the latter communicating under substantially sealed conditions with said cooking vessel, said impregnation vessel having a bath of impregnation liquor therein occupying less than the full interior thereof and the upper level of which is below said pulp materials outlet, said cooking vessel being effectively empty of liquor for vapor cooking, means for moving pulp material through said liquor bath in said impregnation vessel from said pulp materials inlet to said pulp materials outlet and means connected to said outlet for passing said materials to said cooking vessel, means connected to said impregnating vessel above the liquor level for admitting pressurized vapor therein creating in said impregnating vessel above the liquor level and substantially throughout said cooking vessel a pressurized high temperature environment, means defining a flow path for circulating liquor from and to said bath, the respective ends of which flow path both connect with said impregnation vessel to position at locations beneath the level of the liquor bath therein to ob'viateexcessive vapor condensation and consequent dilution of the liquor bath and an increase in the temperature thereof, said flow path having in connection therewith means for acting on the liquor in said path in the course of movement thereof back to said impregnation vessel to remove condensate and/ or contaminants therefrom and means in said flow path for further acting on the liquor therein to establish a required temperature thereof prior to return of the liquor for re-use in said impregnation vessel.

16. A pulp processing system according to claim 15 including conduit means for recycling liquor within said flow path, a heat exchanger means in said conduit means and wherein said means for removing condensate and/or contaminants comprises flash tank means, the conduit means for recycling being connected to the top thereof.

17. A pulp processing system according to claim 16 wherein said flash tank means is incorporated in said flow path to receive therein liquor extracted from said impregnation vessel and there being control means for controlling the amount of liquor passing from said flash tank through said heat exchange means to raise the temperature thereof to provide that on movement back to said flash tank means the excess condensate and/or contaminants are flashed from the liquor prior to return thereof to said impregnation vessel.

18. A process for the production of pulp in which raw pulp material is passed through a bath of conditioning liquid in a pressurized vessel, in a continuing flow, and the material moved from the vessel in an impregnated condition for further treatment, wherein the used conditioning liquid is moved from said vessel, reheated and carried back to said vessel for re-use, characterized by the steps of extracting used conditioning liquid from said bath at a location below its upper level, moving the extracted liquid from said vessel and returning it to said bath adjacent its upper level by way of a flow path exterior to said vessel, in the course of moving the liquid through said flow path diverting extracted liquid from the flow path and in a recycling action thereon subjecting the extracted liquid to selected conditions of temperature and pressure in an environment remote from and controlled independently of the environment of the interior of said vessel to remove therefrom contaminants and/or excess water and then return the resultantly cleansed and/or concentrated liquid to said flow path for movement to said vessel and in the course of its flow to said vessel subsequently heating and/or cooling the cleansed and/or concentrated liquid to the temperature required for said bath whereby to enable the return of extracted liquid at a substantially uniform level of concentration and cleanliness as well as at a controlled temperature.

19. A pulping process as in claim 18 wherein the liquid returned to said vessel is directed to enter the body of said bath immediately below its upper level and the temperature of the returned liquid is maintained at a level to control the desired temperature of said bath and to provide that there is a uniform action by said bath upon the raw material as it is first contacted by the bath upon entering said vessel.

20. A pulping process as in claim -18 characterized by the step of delivering the raw material to said bath through a counterflow of steam at a temperature approximating the boiling point of the moisture contained in the material whereby to gently condition the material to achieve an optimal reaction thereof to the conditioning fluid on contact therewith.

21. A pulping process as in claim 20 characterized by the step of maintaining the body of said bath at a temperature which is lower than the temperature to which the upper level of the bath is subjected.

22. A pulping process as in claim 18 characterized by the steps of directing said impregnated material from the environment of said bath of conditioning liquid to a cooking environment maintained as a vapor bath of substantially higher temperature than that of said liquid bath, continuously passing the material to said cooking environment and in the process extracting and removing excess moisture and liquor while maintaining the material sealed from the atmosphere and under pressure.

23. A method of processing pulp as in claim 18 characterized by the step of delivering raw materials to said bath through a counterflow of steam at a temperature of about 215220 F. and maintaining the impregnating bath at a temperature of about 300 F.

24. A method of processing pulp as in claim 23 characterized by the step of maintaining the level of said bath subject to steam having a temperature in the area of 367 F.

25. A pulping process as in claim 18 characterized by the steps of introducing limited amounts of fresh conditioning liquid to said flow path and in a cycling action thereon during movement through said flow path elevating the temperature thereof and flashing the same to remove excess water and directing the resultant form of the fresh conditioning liquid through said flow path to said vessel together with the cleansed and/ or concentrated liquid resulting from the recycling action on the liquid extracted from the vessel for return thereto.

26. A pulping process as in claim 25 characterized by the further steps of cooking the impregnated chips in a continuing process and then subjecting the same to a fluidizing and fiberizing action while flashing therefrom moisture to produce steam and applying the steam so flashed to produce a medium applied to conditioning the wood chips in the course of their introduction to said bath.

References Cited UNITED STATES PATENTS 3,448,002 6/ 1969' 'Bryce et al. 16219 3,070,156 12/1962 Starrett 162l7 X 3,258,390 6/1966 Tomlinson II 16217 OTHER REFERENCES Britt, Handbook of Pulp and Paper T echnol gy, Reinhold Publishing Corporation (1964), pp. -182.

ROBERT L. LINDSAY, JR., Primary Examiner R. H. TUSHIN, Assistant Examiner US. Cl. X.R. 

